1. Field of the Invention
The present invention relates to Q-switched laser systems, and particularly to Q-switches with high hold off thresholds for high power pulsed lasers and for tunable lasers.
2. Description of Related Art
A typical Q-switched laser oscillator includes a resonant cavity with an output coupler, a gain medium with means for exciting the gain medium to induce laser gain within the resonant cavity, and a Q-switch. The Q-switch operates to hold off lasing action in the cavity in a first state, and to allow lasing action in the cavity in a second state.
One kind of Q-switch which can switch the resonant cavity on and off at very high speed, consists of a polarizer, a Pockels cell, and a quarterwave plate aligned in the resonant cavity next to a high reflecting retro-reflector. In the hold off state, the Pockels cell has no polarization effects on light propagating along the lasing axis. The quarterwave plate in the round trip induces a 90.degree. rotation of the light so that it is blocked by the polarizer in the return trip. In the on state, an electric field is energized in the Pockels cell so that it induces a quarterwave retardation, which effectively unwinds the rotation caused by the quarterwave plate, allowing the beam to pass through the polarizer and lasing action to occur.
The quarterwave plate is used so that the Q-switch is in the hold off state when the electric field applied to the Pockels cell is 0 volts. The quarterwave plate can be eliminated, and in such systems, the hold off state requires the applied electric field to be energized on the Pockels cell to induce quarterwave retardation.
The Pockels cell typically consists of a cylindrical crystal of a material such as KD*P, having ring electrodes mounted on each end of the crystal for inducing a longitudinal electrical field in response to applied voltage. The c-axis of the crystal is aligned with the lasing axis, so it has no polarization dependent effects on the beam under zero electric field. When a field is applied, polarization dependent indices of refraction are induced. When the magnitude of the field is correct, then the crystal behaves as quarterwave plate having induced fast and slow axes.
General information concerning Pockels cell Q-switches is provided in Koechner, Solid-State Laser Engineering, Second Edition, Springer-Verlag (1988) pages 414-431. Information concerning quarterwave plates can be found in Hecht, Optics, Second Edition, Addison-Wesley Publishing Company (1987), pages 303-304.
In typical operation, the laser system is left in the hold off condition in an initial phase of pumping the gain medium. The Q-switch maintains the resonator in a high loss condition, allowing a relatively high amount of stored energy to accumulate in the laser gain medium. At or near the time when the gain medium reaches its peak energy storage level, the Q-switch is switched to the on state by applying a high voltage pulse of appropriate magnitude to offset the phase retardation of the quarterwave plate. This allows the stored energy in the gain medium to generate a high power laser output pulse.
In many laser systems, particularly those with media exhibiting high gains, like Nd:YAG flash lamp pumped lasers, a limiting factor in the amount of Q-switched energy available from the laser is its hold off level. Imperfections in all the major optical components can combine to limit the amount of hold off one can obtain. This condition is also described as the "extinction ratio" of a cavity. In low gain lasers, extinction ratios of 10:1 or even less may be appropriate. However, in high gain systems, ratios of 10.sup.3 -10.sup.4 :1 or more may be required to hold off oscillation of the laser.
It is desirable to have the hold off state correspond to the non-energized Pockels cell. Therefore, most laser systems include the quarterwave plate within the Q-switch. It is found however that it is very difficult to manufacture a quarterwave plate which induces precisely quarterwave retardation at a preferred wavelength. Thus, the quarterwave plate will induce a retardation which is .lambda./4.+-. a significant error. To the extent that there is an error in the quarterwave plate, the Q-switch will allow a small component of the beam to pass through the polarizer. Using current manufacturing techniques, it is difficult to obtain a quarterwave plate which is accurate at a particular wavelength within about 10%-15%.
According to Hecht, it is known that tilting the quarterwave plate about its fast or slow axis allows tuning of the quarterwave plate to a specific frequency in a narrow region about its nominal value. However, because the quarterwave plates are very thin, the tunable region is too narrow to achieve correction for significant errors.
It is desirable to provide Q-switches with high hold off thresholds, or extinction ratios, for use with high power lasers.